
If you're that convinced that your customers are all thieves and liars that you don't trust someone who said they bought a copy and lost it, please make that perfectly clear in your store and people can save themselves the trouble of buying from you in the first place.
Im not convinced at all that all people in sl are thieves and liars. My business partner and i do whatever we can for the customer, there have been times where some have asked for refunds/exchanges/help with fixing or modding something/permission changes and we gladly help in whichever way we can.
Yes there are *SOME* people out there that may want to steal a texture here, or an outfit there, or whatever, but 'One bad apple dont spoil the whole bunch girl' (sorry had to sing that one

Then you have the stores where everything is C/NT and I'll look around and if there are several things I want, I'll leave a polite IM asking if it was possible to get the list of things I like as transfer instead and log back on to a short "If you don't like the permissions, go somewhere else". It should be obvious they don't get a single sale ever for me, and everyone I know that cares about permissions will hear about it.
From what i have seen lately, all the stores that i had frequented in the past have changed permissions to C/NT, and i was unhappy because i like to have NC/T things, but with bugs and things, i can see why they have done so, and i hadnt actually thought that many people were wanting CN/T items. Now i realize that its probably a safer bet in some situations.
If the reply is "Sorry, I just don't do that" it's perfectly fine obviously. Just because anyone asks doesn't mean they have to, just as long as they're polite to a simple polite question.
Whether you make things C/NT or NC/T probably depends on what the majority of your customers prefer, but if the only reason to go with C/NT is because of your own paranoia and you communicate that on to your customers, you're not on the right approach and those people will find better service elsewhere.
For myself and my partner, we try to cater to the customer, because to be completely honest, without the customer, we wouldnt be anything. We mark things one way, but always leave it up for people to I.M us and ask for different permissions on what they may want/need.